RFC 6947

Independent Submission M. Boucadair
Request for Comments: 6947 France Telecom
Category: Informational H. Kaplan
ISSN: 2070-1721 Acme Packet
R. Gilman
Independent
S. Veikkolainen
Nokia
May 2013 The Session Description Protocol (SDP)
Alternate Connectivity (ALTC) Attribute
Abstract
This document proposes a mechanism that allows the same SDP offer to
carry multiple IP addresses of different address families (e.g., IPv4
and IPv6). The proposed attribute, the "altc" attribute, solves the
backward-compatibility problem that plagued Alternative Network
Address Types (ANAT) due to their syntax.
The proposed solution is applicable to scenarios where connectivity
checks are not required. If connectivity checks are required,
Interactive Connectivity Establishment (ICE), as specified in RFC
5245, provides such a solution.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This is a contribution to the RFC Series, independently of any other
RFC stream. The RFC Editor has chosen to publish this document at
its discretion and makes no statement about its value for
implementation or deployment. Documents approved for publication by
the RFC Editor are not a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6947.

1. Introduction
1.1. Overall Context
Due to the IPv4 address exhaustion problem, IPv6 deployment is
becoming an urgent need, along with the need to properly handle the
coexistence of IPv6 and IPv4. The reality of IPv4-IPv6 coexistence
introduces heterogeneous scenarios with combinations of IPv4 and IPv6
nodes, some of which are capable of supporting both IPv4 and IPv6
dual-stack (DS) and some of which are capable of supporting only IPv4
or only IPv6. In this context, Session Initiation Protocol (SIP)
[RFC3261] User Agents (UAs) need to be able to indicate their
available IP capabilities in order to increase the ability to
establish successful SIP sessions, to avoid invocation of adaptation
functions such as Application Layer Gateways (ALGs) and IPv4-IPv6
interconnection functions (e.g., NAT64 [RFC6146]), and to avoid using
private IPv4 addresses through consumer NATs or Carrier-Grade NATs
(CGNs) [RFC6888].
In the meantime, service providers are investigating scenarios to
upgrade their service offering to be IPv6 capable. The current
strategies involve either offering IPv6 only, for example, to mobile
devices, or providing both IPv4 and IPv6, but with private IPv4
addresses that are NATed by CGNs. In the latter case, the end device
may be using "normal" IPv4 and IPv6 stacks and interfaces, or it may
tunnel the IPv4 packets though a Dual-Stack Lite (DS-Lite) stack that
is integrated into the host [RFC6333]. In either case, the device
has both address families available from a SIP and media perspective.
Regardless of the IPv6 transition strategy being used, it is obvious
that there will be a need for dual-stack SIP devices to communicate
with IPv4-only legacy UAs, IPv6-only UAs, and other dual-stack UAs.
It may not be possible, for example, for a dual-stack UA to
communicate with an IPv6-only UA unless the dual-stack UA has a means
of providing the IPv6-only UA with an IPv6 address, while clearly it
needs to provide a legacy IPv4-only device an IPv4 address. The
communication must be possible in a backward-compatible fashion, such
that IPv4-only SIP devices need not support the new mechanism to
communicate with dual-stack UAs.
The current means by which multiple address families can be
communicated are through ANAT [RFC4091] or ICE [RFC5245]. ANAT has
serious backward-compatibility problems, as described in [RFC4092],
which effectively make it unusable, and it has been deprecated by the
IETF [RFC5245]. ICE at least allows interoperability with legacy
devices. But, ICE is a complicated and processing-intensive
mechanism and has seen limited deployment and implementation in SIP
applications.

ALTC has been implemented as reported in [NAT64-EXP]. No issues have
been reported in that document.
1.2. Purpose
This document proposes a new alternative: a backward-compatible
syntax for indicating multiple media connection addresses and ports
in an SDP offer, which can immediately be selected from and used in
an SDP answer.
The proposed mechanism is independent of the model described in
[RFC5939] and does not require implementation of SDP Capability
Negotiations (a.k.a., SDPCapNeg) to function.
It should be noted that "backward-compatible" in this document
generally refers to working with legacy IPv4-only devices. The
choice has to be made, one way or the other, because to interoperate
with legacy devices requires constructing SDP bodies that they would
understand and support, such that they detect their local address
family in the SDP connection line. It is not possible to support
interworking with both legacy IPv4-only and legacy IPv6-only devices
with the same SDP offer. Clearly, there are far more legacy
IPv4-only devices in existence, and thus those are the ones assumed
in this document. However, the syntax allows for a UA to choose
which address family to be backward-compatible with, in case it has
some means of determining it.
Furthermore, even for cases where both sides support the same address
family, there should be a means by which the "best" address family
transport is used, based on what the UAs decide. The address family
that is "best" for a particular session cannot always be known a
priori. For example, in some cases the IPv4 transport may be better,
even if both UAs support IPv6.
The proposed solution provides the following benefits:
o Allows a UA to signal more than one IP address (type) in the same
SDP offer.
o Is backward compatible. No parsing or semantic errors will be
experienced by a legacy UA or by intermediary SIP nodes that do
not understand this new mechanism.
o Is as lightweight as possible to achieve the goal, while still
allowing and interoperating with nodes that support other similar
or related mechanisms.
o Is easily deployable in managed networks.

o Requires minimal increase of the length of the SDP offer (i.e.,
minimizes fragmentation risks).
ALTC may also be useful for the multicast context (e.g., Section 3.4
of [MULTRANS-FW] or Section 3.3 of [ADDR-ACQ]).
More detailed information about ALTC use cases is provided in
Appendix A.
1.3. Scope
This document proposes an alternative scheme, as a replacement to the
ANAT procedure [RFC4091], to carry several IP address types in the
same SDP offer while preserving backward compatibility.
While two UAs communicating directly at a SIP layer clearly need to
be able to support the same address family for SIP itself, current
SIP deployments almost always have proxy servers or back-to-back user
agents (B2BUAs) in the SIP signaling path, which can provide the
necessary interworking of the IP address family at the SIP layer
(e.g., [RFC6157]). SIP-layer address family interworking is out of
scope of this document. Instead, this document focuses on the
problem of communicating media address family capabilities in a
backward-compatible fashion. Because media can go directly between
two UAs, without a priori knowledge by the User Agent Client (UAC) of
which address family the far-end User Agent Server (UAS) supports, it
has to offer both, in a backward-compatible fashion.
1.4. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
2. Use Cases
The ALTC mechanism defined in this document is primarily meant for
managed networks. In particular, the following use cases were
explicitly considered:
o A dual-stack UAC that initiates a SIP session without knowing the
address family of the ultimate target UAS.
o A UA that receives a SIP session request with SDP offer and that
wishes to avoid using IPv4 or IPv6.
o An IPv6-only UA that wishes to avoid using a NAT64 [RFC6146].

o A SIP UA behind a DS-Lite CGN [RFC6333].
o A SIP service provider or enterprise domain of an IPv4-only and/or
IPv6-only UA that provides interworking by invoking IPv4-IPv6
media relays and that wishes to avoid invoking such functions and
to let media go end to end as much as possible.
o A SIP service provider or enterprise domain of a UA that
communicates with other domains and that wishes either to avoid
invoking IPv4-IPv6 interworking or to let media go end to end as
much as possible.
o A SIP service provider that provides transit peering services for
SIP sessions that may need to modify SDP in order to provide
IPv4-IPv6 interworking, but would prefer to avoid such
interworking or to avoid relaying media in general, as much as
possible.
o SIP sessions that use the new mechanism when crossing legacy SDP-
aware middleboxes, but that may not understand this new mechanism.
3. Overview of the ALTC Mechanism
3.1. Overview
The ALTC mechanism relies solely on the SDP offer/answer mechanism,
with specific syntax to indicate alternative connection addresses.
The basic concept is to use a new SDP attribute, "altc", to indicate
the IP addresses for potential alternative connection addresses. The
address that is most likely to get chosen for the session is in the
normal "c=" line. Typically, in current operational networks, this
would be an IPv4 address. The "a=altc" lines contain the alternative
addresses offered for this session. This way, a dual-stack UA might
encode its IPv4 address in the "c=" line, while possibly preferring
to use an IPv6 address by explicitly indicating the preference order
in the corresponding "a=altc" line. One of the "a=altc" lines
duplicates the address contained in the "c=" line, for reasons
explained in Section 3.2. The SDP answerer would indicate its chosen
address by simply using that address family in the "c=" line of its
response.

An example of an SDP offer using this mechanism is as follows when
IPv4 is considered most likely to be used for the session, but IPv6
is preferred:
v=0
o=- 25678 753849 IN IP4 192.0.2.1
s=
c=IN IP4 192.0.2.1
t=0 0
m=audio 12340 RTP/AVP 0 8
a=altc:1 IP6 2001:db8::1 45678
a=altc:2 IP4 192.0.2.1 12340
If IPv6 were considered more likely to be used for the session, the
SDP offer would be as follows:
v=0
o=- 25678 753849 IN IP6 2001:db8::1
s=
c=IN IP6 2001:db8::1
t=0 0
m=audio 45678 RTP/AVP 0 8
a=altc:1 IP6 2001:db8::1 45678
a=altc:2 IP4 192.0.2.1 12340
Since an alternative address is likely to require an alternative
TCP/UDP port number as well, the new "altc" attribute includes both
an IP address and a transport port number (or multiple port numbers).
The ALTC mechanism does not itself support offering a different
transport type (i.e., UDP vs. TCP), codec, or any other attribute.
It is intended only for offering an alternative IP address and port
number.
3.2. Rationale for the Chosen Syntax
The use of an "a=" attribute line is, according to [RFC4566], the
primary means for extending SDP and tailoring it to particular
applications or media. A compliant SDP parser will ignore the
unsupported attribute lines.
The rationale for encoding the same address and port in the "a=altc"
line as in the "m=" and "c=" lines is to provide detection of legacy
SDP-changing middleboxes. Such systems may change the connection
address and media transport port numbers, but not support this new
mechanism, and thus two UAs supporting this mechanism would try to
connect to the wrong addresses. Therefore, this document requires
the SDP processor to proceed to the matching rules defined in Section
4.2.1.

4. Alternate Connectivity Attribute
4.1. ALTC Syntax
The "altc" attribute adheres to the [RFC4566] "attribute" production.
The ABNF syntax [RFC5234] of altc is provided below.
altc-attr = "altc" ":" att-value
att-value = altc-num SP addrtype SP connection-address SP port
["/" rtcp-port]
altc-num = 1*DIGIT
rtcp-port = port
Figure 1: Connectivity Capability Attribute ABNF
The meaning of the fields are as follows:
o altc-num: digit to uniquely refer to an address alternative. It
indicates the preference order, with "1" indicated the most
preferred address.
o addrtype: the addrtype field as defined in [RFC4566] for
connection data.
o connection-address: a network address as defined in [RFC4566]
corresponding to the address type specified by addrtype.
o port: the port number to be used, as defined in [RFC4566].
Distinct port numbers may be used for each IP address type. If
the specified address type does not require a port number, a value
defined for that address type should be used.
o rtcp-port: including an RTP Control Protocol (RTCP) port is
optional. An RTCP port may be indicated in the alternative "c="
line when the RTCP port cannot be derived from the RTP port.
The "altc" attribute is applicable only in an SDP offer. The "altc"
attribute is a media-level-only attribute and MUST NOT appear at the
SDP session level. (Because it defines a port number, it is
inherently tied to the media level.) There MUST NOT be more than one
"altc" attribute per addrtype within each media description. This
restriction is necessary so that the addrtype of the reply may be
used by the offerer to determine which alternative was accepted.
The "addrtype"s of the altc MUST correspond to the "nettype" of the
current connection ("c=") line.

A media description MUST contain two "altc" attributes: the
alternative address and an alternative port. It must also contain an
address and a port that "duplicate" the address/port information from
the current "c=" and "m=" lines. Each media level MUST contain at
least one such duplicate "altc" attribute, of the same IP address
family, address, and transport port number as those in the SDP
connection and media lines of its level. In particular, if a "c="
line appears within a media description, the addrtype and connection-
address from that "c=" line MUST be used in the duplicate "altc"
attribute for that media description. If a "c=" line appears only at
the session level and a given media description does not have its own
connection line, then the duplicate "altc" attribute for that media
description MUST be the same as the session-level address
information.
The "altc" attributes appearing within a media description MUST be
prioritized. The explicit preference order is indicated in each line
("1" indicates the address with the highest priority). Given this
rule, and the requirement that the address information provided in
the "m=" line and "o=" line must be provided in an "altc" attribute
as well, it is possible that the addresses in the "m=" line and "o="
line are not the preferred choice.
If the addrtype of an "altc" attribute is not compatible with the
transport protocol or media format specified in the media
description, that "altc" attribute MUST be ignored.
Note that "a=altc" lines describe alternative connection addresses,
NOT addresses for parallel connections. When several "altc" lines
are present, establishing multiple sessions MUST be avoided. Only
one session is to be maintained with the remote party for the
associated media description.
4.2. Usage and Interaction
4.2.1. Usage
In an SDP offer/answer model, the SDP offer includes "altc"
attributes to indicate alternative connection information (i.e.,
address type, address and port numbers), including the "duplicate"
connection information already identified in the "c=" and "m=" lines.
Additional, subsequent offers MAY include "altc" attributes again,
and they may change the IP address, port numbers, and order of
preference, but they MUST include a duplicate "altc" attribute for
the connection and media lines in that specific subsequent offer. In
other words, every offered SDP media description with an alternative
address offer with an "altc" attribute has two "altc" attributes:

- one duplicating the "c=" and "m=" line information for that
media description
- one for the alternative
These need not be the same as the original SDP offer.
The purpose of encoding a duplicate "altc" attribute is to allow
receivers of the SDP offer to detect if a legacy SDP-changing
middlebox has modified the "c=" and/or "m=" line address/port
information. If the SDP answerer does not find a duplicate "altc"
attribute value for which the address and port exactly match those in
the "c=" line and "m=" line, the SDP answerer MUST ignore the "altc"
attributes and use the "c=" and "m=" offered address/ports for the
entire SDP instead, as if no "altc" attributes were present. The
rationale for this is that many SDP-changing middleboxes will end the
media sessions if they do not detect media flowing through them. If
a middlebox modified the SDP addresses, media MUST be sent using the
modified information.
Note that for RTCP, if applicable for the given media types, each
side would act as if the chosen "altc" attribute's port number was in
the "m=" media line. Typically, this would mean that RTCP is sent to
the port number equal to "RTP port + 1", unless some other attribute
determines otherwise. For example, the RTP/RTCP multiplexing
mechanism defined in [RFC5761] can still be used with ALTC, such that
if both sides support multiplexing, they will indicate so using the
"a=rtcp-mux" attribute, as defined in [RFC5761], but the IP
connection address and port they use may be chosen using the ALTC
mechanism.
If the SDP offerer wishes to use the RTCP attribute defined in
[RFC3605], a complication can arise, since it may not be clear which
address choice the "a=rtcp" attribute applies to, relative to the
choices offered by ALTC. Technically, RFC 3605 allows the address
for RTCP to be indicated explicitly in the "a=rtcp" attribute itself,
but this is optional and rarely used. For this reason, this document
recommends using the "a=rtcp" attribute for the address choice
encoded in the "m=" line and including an alternate RTCP port in the
"a=altc" attribute corresponding to the alternative address. In
other words, if the "a=rtcp" attribute explicitly encodes an address
in its attribute, that address applies for ALTC, as per [RFC3605].
If it does not, then ALTC assumes that the "a=rtcp" attribute is for
the address in the "m=" line, and the alternative "altc" attribute
includes an RTCP alternate port number.

4.2.2. Usage of ALTC in an SDP Answer
The SDP answer SHOULD NOT contain "altc" attributes, because the
answer's "c=" line implicitly and definitively chooses the address
family from the offer and includes it in "c=" and "m=" lines of the
answer. Furthermore, this avoids establishing several sessions
simultaneously between the participating peers.
Any solution requiring the use of ALTC in the SDP answer SHOULD
document its usage, in particular how sessions are established
between the participating peers.
4.2.3. Interaction with ICE
Since ICE [RFC5245] also includes address and port number information
in its candidate attributes, a potential problem arises: which one
wins. Since ICE also includes specific ICE attributes in the SDP
answer, the problem is easily avoided: if the SDP offerer supports
both ALTC and ICE, it may include both sets of attributes in the same
SDP offer. A legacy ICE-only answerer will simply ignore the "altc"
attributes and use ICE. An ALTC-only answerer will ignore the ICE
attributes and reply without them. An answerer that supports both
MUST choose one and only one of the mechanisms to use: either ICE or
ALTC. However, if the "m=" or "c=" line was changed by a middlebox,
the rules for both ALTC and ICE would make the answerer revert to
basic SDP semantics.
4.2.4. Interaction with SDP-Cap-Neg
The ALTC mechanism is orthogonal to SDPCapNeg [RFC5939]. If the
offerer supports both ALTC and SDPCapNeg, it may offer both.
5. IANA Considerations
Per this document, the following new SDP attribute has been assigned.
SDP Attribute ("att-field"):
Attribute name altc
Long form Alternate Connectivity
Type of name att-field
Type of attribute Media level only
Subject to charset No
Purpose See Sections 1.2 and 3
Specification Section 4
The contact person for this registration is Mohamed Boucadair (email:
mohamed.boucadair@orange.com; phone: +33 2 99 12 43 71).

Appendix A. ALTC Use Cases
A.1. Terminology
The following terms are used when discussing the ALTC use cases:
o SBE (Signaling Path Border Element) denotes a functional element,
located at the boundaries of an ITAD (IP Telephony Administrative
Domain) [RFC2871], that is responsible for intercepting signaling
flows received from UAs and relaying them to the core service
platform. An SBE may be located at the access segment (i.e., be
the service contact point for UAs), or be located at the
interconnection with adjacent domains [RFC6406]. An SBE controls
one or more DBEs. The SBE and DBE may be located in the same
device (e.g., the SBC [RFC5853]) or be separated.
o DBE (Data Path Border Element) denotes a functional element,
located at the boundaries of an ITAD, that is responsible for
intercepting media/data flows received from UAs and relaying them
to another DBE (or media servers, e.g., an announcement server or
IVR). An example of a DBE is a media gateway that intercepts RTP
flows. An SBE may be located at the access segment (i.e., be the
service contact point for UAs) or be located at the
interconnection with adjacent domains ([RFC6406]).
o Core service platform ("core SPF") is a macro functional block
including session routing, interfaces to advanced services, and
access control.
Figure 2 provides an overview of the overall architecture, including
the SBE, DBE, and core service platform.

+----------+
| Core SIP |
+--------->| SPF |<---------+
| SIP +----------+ SIP |
v v
+-----------+ +-----------+
+-----+ SIP | SBE | | SBE | SIP
| S |<----->| | | |<----->
| I | +-----------+ +-----------+
| P | || ||
| | +-----------+ +-----------+
| U | RTP | DBE | RTP | DBE | RTP
| A |<----->| |<----------------->| | <----->
+-----+ +-----------+ +-----------+
SIP UA can be embedded in the CPE or in a host behind the CPE
Figure 2: Service Architecture OverviewA.2. Multicast Use Case
Recently, a significant effort has been undertaken within the IETF to
specify new mechanisms to interconnect IPv6-only hosts to IPv4-only
servers (e.g., [RFC6146]). This effort exclusively covered unicast
transfer mode. An ongoing initiative, called "multrans", has been
launched to cover multicast issues that are encountered during IPv6
transition. The overall problem statement is documented in
[MULTRANS-PS].
A particular issue encountered in the context of IPv4/IPv6
coexistence and IPv6 transition of multicast services is the
discovery of the multicast group and source (refer to Section 3.4 of
[MULTRANS-PS]):
o For an IPv6-only receiver requesting multicast content generated
by an IPv4-only source:
* An ALG is required to help the IPv6 receiver select the
appropriate IP address when only the IPv4 address is advertised
(e.g., using SDP). Otherwise, access to the IPv4 multicast
content cannot be offered to the IPv6 receiver. The ALG may be
located downstream of the receiver. As such, the ALG does not
know in advance whether the receiver is dual-stack or
IPv6-only. The ALG may be tuned to insert both the original
IPv4 address and the corresponding IPv6 multicast address
using, for instance, the ALTC SDP attribute.

* To avoid involving an ALG in the path, an IPv4-only source can
advertise both its IPv4 address and its IPv4-embedded IPv6
multicast address [ADDR-FORMAT] using, for instance, the ALTC
SDP attribute.
o For a dual-stack source sending its multicast content over IPv4
and IPv6, both IPv4 and IPv6 addresses need to be inserted in the
SDP part. A means (e.g., ALTC) is needed for this purpose.
A.3. Introducing IPv6 into SIP-Based Architectures
A.3.1. Avoiding Crossing CGN Devices
Some service providers are in the process of enabling DS-Lite
[RFC6333] as a means to continue delivering IPv4 services to their
customers. To avoiding crossing four levels of NAT when establishing
a media session (two NATs in the DS-Lite Address Family Transition
Router (AFTR) and two NATs in the DBE), it is recommended to enable
IPv6 functions in some SBEs/DBEs. Then, DS-Lite AFTRs will not be
crossed for DS-Lite serviced customers if their UA is IPv6-enabled:
o For a SIP UA embedded in the CPE, this is easy to implement since
the SIP UA [RFC3261] can be tuned to behave as an IPv6-only UA
when DS-Lite is enabled. No ALTC is required for this use case.
o For SIP UAs located behind the CPE, a solution to indicate both
IPv4 and IPv6 (e.g., ALTC) is required in order to avoid crossing
the DS-Lite CGN.
A.3.2. Basic Scenario for IPv6 SIP Service Delivery
A basic solution to deliver SIP-based services using an IPv4-only
core service platform to an IPv6-enabled UA is to enable the
IPv4/IPv6 interworking function in the SBE/DBE. Signaling and media
between two SBEs and DBEs is maintained over IPv4. IPv6 is used
between an IPv6-enabled UA and an SBE/DBE.
Figure 3 shows the results of session establishment between UAs. In
this scenario, the IPv4/IPv6 interworking function is invoked even
when both involved UAs are IPv6-enabled.

This section provides an overview of the procedure to avoid IPv4/IPv6
interworking.
When an SBE receives an INVITE, it instantiates in its DBE an
IPv6-IPv6 context and an IPv6-IPv4 context. Both an IPv6 address and
an IPv4 address are returned, together with other information such as
port numbers. The SBE builds an SDP offer, including both the IPv4
and IPv6-related information using the "altc" attribute. IPv6 is
indicated as the preferred connectivity type; see Figure 4.
o=- 25678 753849 IN IP4 192.0.2.2
c=IN IP4 192.0.2.2
m=audio 12340 RTP/AVP 0 8
a=altc:1 IP6 2001:db8::2 6000
a=altc:2 IP4 192.0.2.2 12340
Figure 4: SDP Offer Updated by the SBE
The request is then forwarded to the core SPF, which, in turn,
forwards it to the terminating SBE.
o If this SBE is a legacy one, then it will ignore "altc" attributes
and use the "c=" line.
o If the terminating SBE is IPv6-enabled:
* If the called UA is IPv4 only, then an IPv6-IPv4 context is
created in the corresponding DBE.
* If the called UA is IPv6-enabled, then an IPv6-IPv6 context is
created in the corresponding DBE.
Figure 5 shows the results of the procedure when placing a session
between an IPv4 and IPv6 UAs, while Figure 6 shows the results of
establishing a session between two IPv6-enabled UAs. The result is
still not optimal since redundant NAT66 is required (Appendix A.3.4).

The main advantages of such a solution are as follows:
o DBE resources are optimized.
o No redundant NAT is maintained in the path when IPv6-enabled UAs
are involved.
o End-to-end delay is optimized.
o The robustness of the service is optimized since the delivery of
the service relies on fewer nodes.
o The signaling path is also optimized since no communication
between the SBE and DBE at the terminating side is required for
some sessions. (That communication would be through the Service
Policy Decision Function (SPDF) in a Telecoms and Internet
converged Services and Protocols for Advanced Networks/IP
Multimedia Subsystem (TISPAN/IMS) context.)
A.3.5. Direct Communications between IPv6-Enabled User Agents
For service providers wanting to allow direct IPv6 communications
between IPv6-enabled UAs, when not all SBEs/DBEs and UAs are
IPv6-enabled, a means to indicate both the IPv4 and IPv6 addresses
without inducing session failures is required. Below is an example
of a proposed procedure using the "altc" attribute.
At the SBE originating side, when the SBE receives an INVITE from the
calling IPv6 UA (Figure 9), it uses ALTC to indicate two IP
addresses:
1. An IPv4 address belonging to its controlled DBE.
2. The same IPv6 address and port as received in the initial offer
made by the calling IPv6.
Figure 9 shows an excerpted example of the SDP offer of the calling
UA, and Figure 10 shows an excerpted example of the updated SDP offer
generated by the originating SBE.
o=- 25678 753849 IN IP6 2001:db8::1
c=IN IP6 2001:db8::1
m=audio 6000 RTP/AVP 0 8
Figure 9: SDP Offer of the Calling UA o=- 25678 753849 IN IP4 192.0.2.2
c=IN IP4 192.0.2.2
m=audio 12340 RTP/AVP 0 8
a=altc:1 IP6 2001:db8::1 6000
a=altc:2 IP4 192.0.2.2 12340
Figure 10: SDP Offer Updated by the SBE

The INVITE message will be routed appropriately to the destination
SBE:
1. If the SBE is a legacy device (i.e., IPv4-only), it will ignore
IPv6 addresses and will contact its DBE to instantiate an
IPv4-IPv4 context.
2. If the SBE is IPv6-enabled, it will only forward the INVITE to
the address of contact of the called party:
a. If the called party is IPv6-enabled, the communication will
be placed using IPv6. As such, no DBE is involved in the
data path, as illustrated in Figure 11.
b. Otherwise, IPv4 will be used between the originating DBE and
the called UA.
+----------+
| Core SIP |
+--->|SPF (IPv4)|<---+
IPv4 SIP | +----------+ |IPv4 SIP
v v
+-----------+ +-----------+
| SBE | | SBE | SIP
+------->|IPv4/v6 IWF| |IPv4/v6 IWF|<-------+
|IPv6 +-----------+ +-----------+ IPv6|
| SIP SIP|
+----+ | | +----+
|IPv6|-+ IPv6 RTP +-|IPv6|
| UA |<---------------------------------------------------->| UA |
+----+ +----+
2001:db8::1
Figure 11: Direct IPv6 Communication